Sequential injection molding processes for performing sequential shots of different polymer materials are well known. To accomplish such processes, various apparati have been developed using hotrunner systems that are designed to deliver sequential shots of polymer material to a plurality of cavities. In multicavity applications, shots are intended to be delivered at the same time in the same amounts and at the same rates of flow by controlling the length and configuration of the hotrunner flow channels and the temperature of various portions of the hotrunner and injection nozzles to each cavity. However, in practice, it is very difficult to achieve such uniform delivery to multiple cavities.
When a single source of polymer material is used to effect flow through all channel paths in a hotrunner to multiple mold cavities, the pressure will vary between the flow paths even at points within different channels that are located the same distance (path length) from the source of injection. Still further, changes in the polymer material(s) over time, e.g., different batches, sources, temperatures and moisture content, can alter the flow characteristics of the material and render the system unbalanced.
These types of multi-cavity injection molding systems are in widespread use in the food and beverage industry to make multilayer preforms, which are subsequently blow molded into multilayer containers. In particular, these multilayer preform and container structures enable cost effective use of what are generally more expensive barrier and/or high thermal performance materials, as one or more layers of the preform. Ideally, the amounts of the expensive barrier or high performance polymer materials are utilized in relatively thin layers, thus reducing the overall cost of the preform/container, while a less expensive structural polymer comprises the predominant weight percentage of the article.
For example, Continental PET Technologies (CPT) developed a sequential multilayer injection process for making three-layer or five-layer preforms. A typical five-layer preform includes inner and outer exterior layers of virgin PET, a central core layer of virgin or recycled (e.g., post consumer and/or plant scrap) PET, and two thin intermediate barrier layers between each of the core and exterior layers. A relatively small amount of barrier material, typically 2 to 5 percent of the total preform weight, forms the thin intermediate layers and yet provides effective barrier (e.g., gas, moisture, flavor) performance. In order to provide a uniform and consistent barrier layer structure, the CPT process utilizes devices, commonly referred to as mold shooting (or metering) pots, which comprise a chamber of predefined volume that is filled/prefilled with the polymer material and located adjacent to an associated cavity. This enables injection of a precise amount of virgin PET (greater than 50% of the total preform weight) from a first shooting pot during a first shot injection, followed by a precise amount of barrier material (2-5% of the total preform weight) from a second shooting pot during a second shot injection. A third shot of virgin or post-consumer PET is injected from a machine shooting pot (the first, second and third shots comprising approximately 95% of the total preform weight). A fourth and final shot of virgin PET is then injected from a machine shooting pot to pack the preform, and clean out the post-consumer PET from the nozzle in preparation for the next cycle. The CPT process and multilayer articles are described in one or more of U.S. Pat. Nos. 4,550,043; 4,609,516; 4,710,118; 4,781,954; 4,950,143; 4,990,301; 4,923,723; and 5,098,274, the disclosures of all of which are incorporated herein by reference as if fully set forth herein.
The use of mold shooting pots is an effective way to provide precise amounts of polymer material for the various layers and insure consistent preform layer structure. However, there is a cost associated with utilizing shooting pots in multicavity systems, namely the associated cost of providing a shooting pot for each material for each mold cavity and the expense in providing sufficient physical space in the machine platen to accommodate all of these shooting pots. As a result of the increased demand for platen space, the preform manufacturer is generally required to purchase a larger more expensive, higher tonnage machine, even though the increased tonnage is not required. Also, when multiple shots of polymer material are affected using metering pots, the sequence and timing of the shots becomes cumbersome and more time-consuming in having to complete all shots of all polymer materials from multiple metering pots mounted in multiple locations on a hotrunner/manifold.
For these reasons, others have attempted to rely on balanced manifolds for delivering each of the multiple layer materials. However, while avoiding the cost and space constraints of shooting pots, these systems do not consistently produce uniform layer structures in the preform.
It would thus be desirable to provide alternative injection molding systems for forming multilayer articles in multiple cavities, particularly in the manufacture of multilayer preforms utilizing relatively low weight percentages of select (e.g., barrier layer) materials but requiring formation of a consistent layer structure across multiple cavities.